The present invention relates to an electronic navigation apparatus, and further relates to a method of operating such an apparatus.
A variety of electronic navigation systems are known. All work by having a plurality of radio transmitters, each transmitting information about its respective position, this information enabling a receiver to determine its position from the received signals. Currently available systems include GPS (Global Positioning System), a US satellite-based system, and GLONASS (GLObal NAvigation Satellite System), the Russian equivalent.
Receivers for such systems are used in a variety of applications, including boats and aircraft. An increasingly popular application is in electronic guidance systems for vehicles, where the electronic navigation system is used in conjunction with an electronic map to direct the vehicle driver to their desired destination.
GPS is a widely used system and comprises a constellation of typically 24 satellites in six inclined, approximately 12-hour circular orbits around the earth. Each satellite carries extremely accurate atomic clocks, and transmits a uniquely coded spread spectrum signal on a carrier frequency centred at 1.575 GHz which provides information about the current time and location of the satellite. There are coarse and fine versions of the spread spectrum signals, the former being for civilian usage and the latter for military applications. Reception of coarse signals from four or more satellites provides sufficient information for the receiver to be able to determine its three-dimensional position on (or near) the earth""s surface and the current time. The satellite orbits are arranged so that there are always at least four satellites visible at any point on the earth""s surface, unless the receiver""s view of the sky is blocked by buildings or other obstructions.
As well as determination of the receiver""s position, derived from the time of arrival of each of the satellite signals, the receiver""s velocity can be determined from the Doppler shift in the frequency of the transmission from each satellite.
The nature of the transmitted signal gives rise to random fluctuations in the apparent position and velocity of the receiver. This fluctuation is undesirable, and some filtering mechanism is generally used before the position and velocity are reported to the user of the receiver. In such a process, information about previous position and velocity of the receiver is combined with new measurements to arrive at a statistically acceptable result. This process takes into account both the confidence in the old results and the quality of the new measurements. It also enables the receivers position to be extrapolated for short periods when insufficient satellites are visible for accurate positional fixes to be made, based on the assumption of constant velocity.
Information about the acceleration of the receiver can further improve the accuracy of the filtering process and its immunity to noise. At present, such information can only be obtained from mechanical accelerometers (effectively one or more masses on springs), which cannot be conveniently combined with an integrated circuit. A discussion of how to integrate a GPS receiver and an inertial navigation system including accelerometers, indicating some of the complexities involved, is provided in xe2x80x9cUnderstanding GPS: Principles and Applicationsxe2x80x9d, page 395, E D Kaplan, Artech House, 1996.
An object of the present invention is to improve the determination of acceleration in an electronic navigation apparatus.
According to a first aspect of the present invention there is provided an electronic navigation apparatus comprising first and second tracking channels, the first tracking channel having a carrier tracking loop arranged to track accurately the frequency of a remote transmitter as received, the second tracking channel having a tracking loop arranged to track less accurately under dynamic conditions the frequency of the remote transmitter as received, and means for combining the accurately and less accurately tracked frequencies to provide a measure of a kinematic parameter of the navigation apparatus relative to the remote transmitter.
Advantageously, to determine the order n kinematic parameter (where n=3 for acceleration, n=4 for jerk, etc) the accurate carrier tracking loop is implemented as a phase locked loop of order n, and the less accurate carrier tracking loop is implemented as a frequency locked loop of order nxe2x88x922 or a phase locked loop of order nxe2x88x921.
According to a second aspect of the present invention there is provided a method of operating an electronic navigation apparatus comprising first and second tracking channels, the first tracking channel having a carrier tracking loop arranged to track accurately the frequency of a remote transmitter as received, the second tracking channel having a tracking loop arranged to track less accurately under dynamic conditions the frequency of the remote transmitter as received, the method comprising combining the accurately and less accurately tracked frequencies to provide a measure of a kinematic parameter of the navigation apparatus relative to the remote transmitter.
The present invention is based upon the recognition, not present in the prior art, that targeting a plurality of carrier tracking loops having different dynamic tracking properties on a remote transmitter enables acceleration and higher order kinematic parameters to be derived.
By means of the present invention determination of acceleration and higher order kinematic parameters directly from the received transmissions by electronic circuitry is enabled.